Gold is usually found in ore bodies, or in waterborne or alluvial deposits that have collected and concentrated gold by physical processes. These processes include gold grains rolling down a stream bed.

Scientists know that microorganisms are involved in dissolving trace amounts of gold out of rock, which Reith's research confirmed.

Gold in the quartz vein at the Tomakin mine on the south coast of New South Wales is invisible, even to high-powered electron microscopes, said Reith.

The gold is hidden in sulfur-rich metal minerals, until the microbes remove it from the rock atom by atom.

But Reith suggested the microbes played another important role: transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits.

He found traces of the microbes' genetic material on the deposits, which confirmed they were present.

Reith then took some of the microbes from the mine and used them to 'grow' gold in the lab.

Reith said the microbes at the surface of these formations could still be hard at work, forming new gold layers on top of the past generation's fossilised gold deposit.

Marker for gold

Reith suggested that the presence of the gold-digging microbes could be used as an environmentally friendly marker for geological exploration.

Looking at past geophysical data for signs of the microbes could prompt further exploration of an area where there is no visible gold deposit, he said.

"It would never substitute for other methods but it could be an add-on," said Reith. "If you found a lot of these organisms in the field you [would be able to] tell that there is some sort of mineralisation there."

Reith said the next step in his research would be to extract genetic material from the microbes to identify the species. He suspects they belong to the genus Pedomicrobium, which contains microbes known to dissolve other metals.

"It's very hard to pinpoint certain organisms; there are millions growing in the soil," he said.

Dr Dennis Gee, chief executive of the research centre involved in the research, said the process Reith unearthed could apply to the main alluvial gold fields in Victoria, at Bathurst in New South Wales, in the Northern Territory and possibly at Coolgardie in Western Australia.

The research could also be important in processing gold from ores that are hard to dissolve, said Gee. These make up one-third of all Australia's gold reserves.